This application is a continuation-in-part of U.S. application Ser. No. 158,478, filed Feb. 22, 1988, of Stuart I. Yaniger and Randy E. Cameron, and assigned to the same Assignee as the present application.
This invention relates to the production of electrically conductive polymer materials and is particularly concerned with the solution blending of conductive polyaniline and conductive polyaniline derivatives, with maleimide systems, particularly bismaleimide, to produce cured maleimide materials having electrical conductivity, without decreasing the mechanical properties of the maleimide component.
The free-base form of polyaniline is believed to comprise subunits having the formula: ##STR1## where n is between 0 and 1. The oxidation state of polyaniline referred to as "emeraldine" is believed to have a value of n of about 0.5.
This free-base form of polyaniline is an electrical insulator. Reaction of emeraldine free-base with protonic acids of the form HX, where X is, for example, Cl, causes the polymer to undergo an insulator to conductor transition, as disclosed in A. G. MacDiarmid, ct al, Mol. Cryst. Liq. Cryst. 121, 173 (1985). Conductive polyaniline of this type has been employed in batteries as disclosed, for example, in French Patent No. 1,519,729.
However, a number of difficulties have been encountered with the prior art materials noted above. Thus, the conductive polyaniline acid salts are, with a few exceptions, insoluble in most solvent media. None of the polyanilines can be melted. The emeraldine free-base and the conductive forms thereof noted above tend to form powders on removal of the solvent. With some effort, films can be cast; however, they are quite fragile and brittle, easily crumbling to form a powder. The conductive acid salts lose their conductivity when exposed to liquid water. This loss is due to deprotonation. The conductivity loss is reversible; treatment of the deprotonated material with protic acids restores the conductivity. Further, conductive regions in an insulating matrix tend toward diffusion. For example, if one makes a conductive trace of polyaniline acid salt on a substrate of emeraldine free-base, the trace remains spatially stable for only a short time, eventually spreading out until the substrate has a constant conductivity throughout.
Some of these problems were addressed in U.S. applications Ser. No. 920,474 filed Oct. 20, 1986, of S. I. Yaniger, and Serial No. 013,305 filed February 11, 1987, of S. I. Yaniger, et al, both assigned to the same assignee as the present application. In these applications, it is disclosed that Lewis acids, for example, alkylating agents, can be used to make the insulating emeraldine free-base into a conductive polymer salt. Use of proper Lewis acids resulted in conductive polyanilines with the Lewis acid as a side chain. These derivatized polyanilines are more water stable and processable than the prior art emeraldine acid salts. Additionally, no diffusion between "doped" conducting and "undoped" insulating regions was observed.
Thus, in the above U.S. application, Ser. No. 920,474, a base-type non-conductive polymer, such as polyaniline, can be reacted with, for example, methyl iodide, to form an electrically conductive polymer in which the methyl group is covalently linked to the nitrogen atoms of the polymer.
In the above U.S. application, Ser. No. 013,305, emeraldine free-base can be reacted with reagents of the form RSO.sub.2 Cl, e.g., tosyl chloride, to form an electrically conductive polymer in which the --SO.sub.2 R groups are covalently linked to the nitrogen atoms of the polymer.
U.S. application Ser. No. 158,477 filed Feb. 22, 1988, of S. I. Yaniger and R. E. Cameron and assigned to the same assignee as the present application, discloses reaction of a base-type non-conductive polymer, such as polyaniline, with an anhydride, such as tosylic anhydride or benzophenone tetracarboxylic dianhydride, and forming an electrically conductive polymer in which the --SO.sub.2 R and --COR groups are covalently linked to the nitrogen atoms of the conductive polymer.
In general, however, the conductive polymers of the above applications tend to be brittle, resulting in inferior mechanical properties.
It would be desirable to blend the relatively brittle conducting polymer with a flexible polymer to form a blend having both the desired electrical properties and good flexibility.
To achieve high electrical conductivity the proportion of conductive polymer to non-conductive polymer in the blend must be relatively high (e.g., greater than 50%) in order for charge to be transferred effectively between polymer chains. Unfortunately, at high polyaniline loadings, the blend materials tend to phase separate, that is, the polyaniline aggregates into clumps within the non-conductive polymer matrix. These clumps are separated by the matrix material, and the blend thus is an insulator. Further, the mechanical properties of the material suffer upon phase separation. It would be desirable to form blends where the polyaniline is dispersed evenly on a molecular level at all loadings, to thus form a conductive polymer blend.
In the above U.S. application Ser. No. 158,478, of which the present application is a continuation-in-part, there is disclosed a conductive polymer blend formed by first reacting a base-type non-conductive polymer containing carbon-nitrogen linkages, such as polyaniline, with a carbonyl anhydride, such as 3,3',4,4'-benzophenone tetracarboxylic dianhydride, to form a conductive polymer containing polyimide-like groups covalently linked to nitrogen atoms of the base-type polymer, mixing such conductive polymer with non-conductive polyimide in a suitable solvent, removing the solvent, and forming a conductive continuous phase blend of the polyimide and the conductive polymer. However, unless the polyimide has a very low melt temperature, the conductive polymer-polyimide blends of the above application are not melt processible and are more useful for making conductive films or fibers than large parts, in which a meltable resin is necessary. In order for a conductive polyaniline to be melt processed or cured with another resin system, such polyaniline must be able to withstand the curing temperature of the other resin system.
In U.S. application Ser. No. 226,484, filed Aug. 1, 1988, by R. E. Cameron, and assigned to the same assignee as the present application, there is disclosed conductive multisulfonic acid derivatives of polyaniline which are highly thermally stable.
Examples of other conductive polymer mixtures are set forth in the following patents.
U.S. Pat. No. 4,526,706 to Upson, et al, discloses a conductive latex coating composition useful in forming conductive layers which comprises a latex having as a dispersed phase in water hydrophobic polymer particles having associated therewith a polyaniline salt semiconductor. The preferred polymer particles are polyurethane particles, but other polymer particles, such as various acrylate polymers, can be employed.
U.S. Pat. No. 3,766,117 to McQuade discloses a method of preparing an electrodepositable solution of a polyamic acid in an organic solvent, for use in electrodepositing a polyamic acid coating on an electrically conducting substrate. The method comprises preparing a solution of an aromatic polyamic acid in an organic solvent, adding to the polyamic acid solution a base, such as ammonia or an organic amine, e.g., an alkanolamine, and adding water to the base-modified polyamic solution to precipitate at least a portion of the polyamic acid to form a stable electrodepositable dispersion of polyamic acid. A coating of polyamic acid is then electrodeposited from the medium onto a conductive substrate, and the coating is then cured to a polyimide to form an insulation coating.
An object of the present invention is the provision of improved electrically conductive polymer materials of the class of conductive polyaniline blended with an imide other than the polyimides of the above U.S. application Ser. No. 158,478.
Another object is to provide conductive polymer materials having improved flexibility, mechanical properties, and thermal stability in the form of a continuous phase blend of a conductive polymer, e.g., conductive polyaniline, and a maleimide, which is capable of melt processing and is particularly applicable for production of large parts.
A still further object is to render imides, particularly bismaleimides, conductive by doping with a conductive polymer, such as conductive polyaniline, to produce an easily processable, highly thermally stable conductive polymer blend.
A still further object is to provide novel procedure for blending polyaniline in the solution phase with a maleimide such as bismaloimide, whereby on removal of the solvent, the resulting polymer blend can be processed to yield strong adhesive conductive resins.